1. Field of the Invention
The present invention is generally directed to subsea completion systems for oil and gas wells, and, more particularly, to, in one embodiment, a subsea system comprising a top flow Christmas tree.
2. Description of the Related Art
A typical subsea well comprises a high pressure wellhead housing which supports one or more casing hangers located at the upper ends of strings of casing that extend into the well. The system further comprises a tubing hanger that supports a string of production tubing through which the oil and/or gas products will eventually be produced. Such a system further comprises a production tree or Christmas tree, e.g., a horizontal or vertical Christmas tree, that contains one or more production bores and a plurality of actuatable valves to control the flow of fluids through the production tree.
Conventionally, wells in oil and gas fields are built up by establishing a wellhead housing, and with a drilling blow out preventer stack (BOP) installed, drilling down to produce the well hole while successively installing concentric casing strings, which are cemented at the lower ends and sealed with mechanical seal assemblies at their upper ends. In order to convert the cased well for production, a tubing string is run in through the BOP and a hanger at its upper end landed in the wellhead. Thereafter the drilling BOP stack is removed and replaced by a Christmas tree having one or more production bores containing actuatable valves and extending vertically to respective lateral production fluid outlet ports in the wall of the Christmas tree.
Such an arrangement introduces many problems which have, previously, been accepted as inevitable. For example, any operations down hole have been limited to tooling which can pass through the production bore, which is usually no more than five inch diameter, unless the Christmas tree is first removed and replaced by a BOP stack. However this involves setting plugs or valves, which may be unreliable by not having been used for a long time, down hole. The well is in a vulnerable condition while the Christmas tree and BOP stack are being exchanged and neither one is in position, which is a lengthy operation. Also, if it is necessary to pull the completion, consisting essentially of the tubing string on its hanger, the Christmas tree must first be removed and replaced by a BOP stack. This usually involves plugging and/or killing the well.
Achieving proper alignment among the various components of a completion system for a subsea well can be a very difficult and time-consuming task. The various components of a subsea completion system, e.g., wellhead, Christmas tree, tubing hanger, etc., are arranged in a stacked configuration wherein each of the various components must be oriented relative to one another or to a fixed reference point, e.g., the wellhead or a guide base. Such orientation is required to insure that the various components properly interface with one another, and to insure that the production outflow line is properly directed toward another subsea component, e.g., a manifold, located on the sea floor. More specifically, proper angular alignment is required to insure that various fluid flow bores and electrical and/or hydraulic lines properly interface with one another when the various components and emergency disconnect devices are stacked up. A very high degree of accuracy, e.g., ±2 degrees, is required in orienting the various components to one another and relative to other subsea components. Such precise alignment is necessary if proper connections are to be made without damage as the devices are lowered into engagement with one another.
This orientation problem is exacerbated in the case of subsea wells as the various devices which are to be stacked up are run down onto guide posts or a guide funnel projecting upwardly from a guide base. The post receptacles which ride down on to the guide posts or the entry guide into the funnel do so with appreciable clearance. This clearance inevitably introduces some uncertainty in alignment and the aggregate misalignment when multiple devices are stacked, can be unacceptably large. Also the exact orientation will depend upon the precise positions of the posts or keys on a particular guide base and the guides on a particular running tool or BOP stack and these will vary significantly from one to another. Consequently it is preferable to ensure that the same running tools or BOP stack are used for the same wellhead, or a new tool or stack may have to be specially modified for a particular wellhead. Further misalignments can arise from the manner in which the guide base is bolted to the conductor casing of the wellhead. As is clear from the foregoing, achieving proper orientation of the various components that comprise a subsea production system can be a very difficult, expensive and time-consuming task.
The present invention is directed to an apparatus and methods for solving, or at least reducing the effects of, some or all of the aforementioned problems.